Description

D = fdesign.highpass(SPEC) constructs
object D and sets the Specification property
to SPEC. Entries in the SPEC string
represent various filter response features, such as the filter order,
that govern the filter design. Valid entries for SPEC are
shown below. The strings are not case sensitive.

Note:
Specifications strings marked with an asterisk require the DSP System Toolbox™ software.

'Fst,Fp,Ast,Ap' (default spec)

'N,F3db'

'N,F3db,Ap' *

'N,F3db,Ast' *

'N,F3db,Ast,Ap' *

'N,F3db,Fp *

'N,Fc'

'N,Fc,Ast,Ap'

'N,Fp,Ap'

'N,Fp,Ast,Ap'

'N,Fst,Ast'

'N,Fst,Ast,Ap'

'N,Fst,F3db' *

'N,Fst,Fp'

'N,Fst,Fp,Ap' *

'N,Fst,Fp,Ast' *

'Nb,Na,Fst,Fp' *

The string entries are defined as follows:

Ap — amount of ripple allowed
in the pass band in decibels (the default units). Also called Apass.

Ast — attenuation in the
stop band in decibels (the default units). Also called Astop.

F3db — cutoff frequency
for the point 3 dB point below the passband value. Specified in normalized
frequency units.

Fc — cutoff frequency for
the point 6 dB point below the passband value. Specified in normalized
frequency units.

Fp — frequency at the start
of the pass band. Specified in normalized frequency units. Also called
Fpass.

Fst — frequency at the end
of the stop band. Specified in normalized frequency units. Also called
Fstop.

N — filter order.

Na and Nb are
the order of the denominator and numerator.

Graphically, the filter specifications look similar to those
shown in the following figure.

Regions between specification values like Fst and Fp are
transition regions where the filter response is not explicitly defined.

The filter design methods that apply to a highpass filter specification
object change depending on the Specification string.
Use designmethods to determine
which design method applies to an object and its specification string.

Use designopts to determine
which design options are valid for a given design method. For detailed
information on design options for a given design method, METHOD,
enter help(D,METHOD) at the MATLAB® command
line.

D = fdesign.highpass(specvalue1,specvalue2,specvalue3,specvalue4) constructs an object D with
the default Specification property and the values
you enter for specvalue1,specvalue2,....

D = fdesign.highpass(...,Fs)
provides the sampling frequency for the filter specification object. Fs is
in Hz and must be specified as a scalar trailing the other numerical
values provided. If you specify a sampling frequency, all other frequency
specifications are in Hz.

D = fdesign.highpass(...,MAGUNITS) specifies
the units for any magnitude specification you provide in the input
arguments. MAGUNITS can be one of

'linear' — specify the magnitude
in linear units

'dB' — specify the magnitude
in dB (decibels)

'squared' — specify the
magnitude in power units

When you omit the MAGUNITS argument, fdesign assumes
that all magnitudes are in decibels. Note that fdesign stores
all magnitude specifications in decibels (converting to decibels when
necessary) regardless of how you specify the magnitudes.

Examples

Highpass filter a discrete-time signal consisting of two sine
waves.

Create a highpass filter specification object. Specify the passband
frequency to be 0.25π radians/sample and the stopband frequency
to be 0.15π radians/sample. Specify 1 dB of allowable passband
ripple and a stopband attenuation of 60 dB.

Create an FIR equiripple filter and view the filter magnitude
response with fvtool.

Hd = design(d,'equiripple');
fvtool(Hd);

Create a signal consisting of the sum of two discrete-time sinusoids
with frequencies of π/8 and π/4 radians/sample and amplitudes
of 1 and 0.25 respectively. Filter the discrete-time signal with the
FIR equiripple filter object, Hd

If you have the DSP System Toolbox software, you can specify
the shape of the stopband and the rate at which the stopband decays.

Create two FIR equiripple filters with different linear stopband
slopes. Specify the passband frequency to be 0.3π radians/sample
and the stopband frequency to be 0.35π radians/sample. Specify
1 dB of allowable passband ripple and a stopband attenuation of 60
dB. Design one filter with a 20 dB/rad/sample stopband slope and another
filter with 40 dB/rad/sample.